Natural rubber hardening

Natural rubber of consistent Mooney viscosity storage hardening has been inhibited by incorporating a small quantity of a compound which reacts with the aldehydic groups present in the rubber. 

The presence of a comonomer has, in certain cases, 9 marked influence on polymerization rate. For example, the mastication of natural rubber in the presence of maleic anhydride, even with small concentrations of the latter, about 5%, leads to accelerated polymerization of styrene monomer (11) either because of its high reactivity in the propagation step of heterochain copolymerization and/or because of a hardening effect. This reaction is discussed later. 

A cure which is longer than optimum is over cure. Over cures may be of two types. In one type the rubber continues to harden, the modulus rises and tensile strength and elongation fall. In certain cases, including most natural rubber compounds, reversion occurs with over cure leading to decrease in tensile strength and modulus. 

Rubbers, or elastomers, stretch readily to elongate by a factor of 10 before breaking. Natural rubber, obtained from the sap of certain trees, can be hardened and toughened by addition of sulfur in the vulcanization process. Synthetic rubber is produced by addition copolymerization of butadiene and styrene. 

Finite chain extensibility is the major reason for strain hardening at high elongations (Fig. 7.8). Another source of hardening in some networks is stress-induced crystallization. For example, vulcanized natural rubber (cw-polyisoprene) does not crystallize in the unstretched state at room temperature, but crystallizes rapidly when stretched by a factor of 3 or more. The extent of crystallization increases as the network is stretched more. The amorphous state is fully recovered when the stress is removed. Since the crystals invariably have larger modulus than the surrounding... 

Natural rubber is produced from a milky-white colloidal latex found in the rubber tree. It is a polymeric terpene with isoprene being the recurring polymeric unit. Polyisoprene rubber can also be produced synthetically by the addition polymerization of isoprene by 1,4-addition. Other synthetic rubbers include SBR (styrene-butadiene rubber), polybutadiene, and neoprene. Rubber is strengthened, hardened, and made more elastic by a process called vulcanization in which sulfur bridges form links within the polymeric chains. These links become strained when the rubber is stretched and when released the rubber assumes its original conformation. 

Vulcanize Heat treatment to harden natural rubber. 

Figure 11.6 Natural rubber, which is soft and very sticky, is hardened in a chemical process called vulcanization. During vulcanization, molecules become linked together, forming a durable material that is harder, smoother, and less sticky. These properties make vulcanized rubber ideal for many products, such as this caster. 

M.J. Gregory, and A.S.Tan, Some observations on storage hardening of Natural Rubber, Proc. Int. Rubber Conf., Kuala Lumpur, Vol. IV, p. 28,1975. 

Surface and bulk effects have been noted in numerous polymer/tritium studies. In one study, hardening of neoprene occurred throughout the bulk, while hardening of natural rubber primarily occurred at the surface (crack propagation). Total incorporation of tritium into a polyethylene powder was found not to be a function of the amount of powder, but of the exposed surface area. Radiation-induced fluorescence from the surface of high-density polyethylene exposed to tritium was shown to be orders of magnitude greater than that from the bulk. 

A linear-chained epoxy resin was formulated from phenyl glycidyl ether and nadic methyl anhydride, catalysed by benzyldimethylamine (248). An IR fibre-optic probe was used to follow the conversion of a thermosetting tetrafunctional epoxy resin in which the hardener was an aromatic diamine and a carboxylic dianhydride. A polymerisation system consisting of a cycloaliphatic diepoxide, epoxidised natural rubber (ENR), glycidyl methacrylate (GMA) and a cationic photoinitiator, triphenylsulfonium hexafluoro-antimonate, was studied (75). Multifunctional epoxy/ amine formulations (Epon 825 plus 4,4 -methylene-... 

Natural rubber does not have as satisfactory a resistance to fuels and vegetable and animal oils as elastomeric thermoplastics and synthetic rubbers. Natural rubber has good resistance to acids and alkalis. It is soluble in aliphatic, aromatic, and chlorinated solvents, but natural rubber does not dissolve easily because of its high molecular weight. Synthetic rubbers have better aging properties. Synthetic rubbers will harden over time, whereas natural rubbers will soften over time. 

Allergic contact dermatitis can be caused by epoxyresin compounds (Tarvainen et al. 1995), UP resin (Liden et al. 1984 Tarvainen et al. 1993a, 1995) and chemicals used as accelerators (cobalt) (Bourne and Milner 1963 Tarvainen et al. 1993b, 1995) or hardeners (organic peroxides) (Bourne and Milner 1963), as well as by p-tert-butylphenol formaldehyde resin (Tarvainen et al. 1993b) and various woods (Hausen and Adams 1990). Natural rubber latex in protective gloves has caused occupational contact urticaria (Tarvainen et al. 1993b). 

Hardener in epoxy resin systems Phenyl glycidyl ether, 0.25% petrolatum. Reactive diluent in epoxy resin based adhesives Triethanolamine, 2.5% petrolatum. In natural rubber glues... 

Natural rubber is a constituent of the latex of Hevea brasiliensis a tree grown in the tropics. The latex is present in laticiferous vessels and can be collected after incision of the bark. It contains about 30 % of rubber in form of small droplets and coagulates after the addition of acids. The coagulate is processed to bandlike sheets of raw" rubber (smoked sheets, crepes) which in the industry are heated with sulfur (vulcanization) to harden it. At present time about half of the rubber used in the industry is prepared from Hevea latex, the other half is of synthetic origin. 

Crystallization. Some elastomers crystallize at temperatures that can significantly impact processing and vulcanizate behavior. Thus it is necessary to account for these behaviors when developing rubber compound formulations and processes. Crystallization is manifested by stiffening and hardening of the raw polymer, uncured compounded polymer, and the vulcanized polymer. Elastomers that crystallize will do so on stretching and thereby exhibit increased tensile strength. Those elastomers (eg polychloroprene and natural rubber) will... 

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